Hmm, realy wierd because when i deactivate wpForo it is all ok. And using wordpress healt check plugin built by automattic show this issue only on wpForo.
This is related to wpForo in terms of conflict. There is some conflict with wpForo, so you see this problem when wpForo is active. But this is not wpForo core error. This is want I mean.
I recommend deactivate other plugins, especially the Yoast and test it with only wpForo. I',m sure there should not be any problem. Also I recommend install WP crone-job manager plugin, remove all crone-job and let plugins initiate those again.
Well, i have done all the tests you have pointed out and everything points against wpForo so then i guess it must be a conflict between theme and wpForo as i have not tested with another theme.
I have deactivated WP cron jobs and setup these in my server so its no problem for me any longer, everything works perfect.
Spending some time searching for this problem (edit theme and extensions timeout in my case), it seems that this came from the 'sandboxing' features that cause this issue were introduced last year to WP.
And disabling wpForo solves that.
Still my CRONs seem to work allright.